Digitizers are used to provide signals respectively representing the rectilinear coordinates of the position of a pointer as it is moved over a planar pad having two grids of parallel insulated conductors mounted thereon in such manner that the conductors of one grid are perpendicular to the conductors of the other. The signals can be used for a number of purposes including the control of the position of a spot of light known as a cursor on the screen of a cathode ray tube. The signals can also be recorded as the pointer traces the line of a drawing, thereby making it possible to reproduce the drawings on a computer aided drawing instrument. Electrical interaction between the pointer and the conductors of one grid is used to provide the signal for the position of the pointer along a coordinate perpendicular to the conductors of that grid, and electrical interaction between the pointer and the conductors of the other grid is used to provide the signal for the position of the pointer along a coordinate that is perpendicular to those conductors.
Two modes of operation are used. In one, electrical stimuli are applied to the pointer and the responses of a grid of conductors provides the data from which a coordinate signal is derived, and in the other, electrical stimuli are applied to the conductors and the responses of the pointer provide the data from which a coordinate signal is derived. In either case, the response characteristic, which is a plot of the responses associated with conductors on either side of the pointer, will be the same.
Most of the digitizers determine the position of the pointer along a rid of conductors by obtaining the response for successive conductors and noting when the response passes through a transition such as a change in phase or a change from a positive amplitude to a negative amplitude. Only the conductors on either side of the transition have significant effect so that the signals representing the coordinate position of the pointer are subject to noise.
Another disadvantage of prior digitizers is that their control systems can only operate with a pointer-grid conductor combination having a given response characteristic, thereby limiting the design of the pointer and grid conductor that can be used.